Answer
Each $Mg$ atom has an oxidation number equal to $+2$, and each $N$ has an oxidation number equal to $-3$.
Work Step by Step
According to rule 6(c), $Mg$ has an oxidation number equal to $+2$.
According to rule 3, the sum of the oxidation numbers must be equal to 0.
Since we have $Mg_3N_2$, and we know that $Mg$ has $+2$, let's call the oxidation number for nitrogen "x":
(+2) * 3 + (x) * 2 = 0
6 + 2x = 0
2x = -6
x = -3
The oxidation number of the nitrogen atoms in this compound is equal to $-3$.
